Method of preparing slow release
fertilizer compositions

ABSTRACT

A METHOD FOR MAKING A SLOW RELEASE EXTRUDED COMPRESSED PELLETIZED FERTILIZER COMPOSITION COMPRISING AT LEAST 75% BY WEIGHT OF A SUITABLE FERTILIZER SUBSTANCE AND LESS THAN 15% BY WEIGHT OF A WATER SOLUBLE HYDROCARBON BINDER MATERIAL, SAID BINDER AND FERTILIZER BEING FURTHER ENCAPSULATED WITH EITHER MICROCRYSTALLINE WAX, ASPHALT, OR A MIXTURE OF WAX AND ASPHALT SO AS TO PROVIDE A COATING COMPLETELY AROUND THE OUTSIDE OF THE FERTILIZER SUBSTANCE. SUCH PELLETS EXHIBIT AN IMPROVED LEACHING RATE OF THE FERTILIZER SUBSTANCE TO THE SURROUNDING AREA OVER SIMILAR COMMERICAL MATERIALS.

United States Patent 27,238 METHOD OF PREPARING SLOW RELEASE FERTILIZER COMPOSITIONS Robert L. Stansbury, Littleton, Colo., and Charles S. Lynch, Lebanon, and Kamil Sor, Linden, N.J., assignors to Esso Research and Engineering Company Original No. 3,276,857, dated Oct. 4, 1966, Ser. No. 401,672, Oct. 5, 1964, which is a continuation of Ser. No. 719,794, Apr. 2, 1968. Application for reissue Jan. 16, 1970, Ser. No. 3,534

Int. Cl. C051? 11/00 U.S. C]. 71-27 4 Claims Matter enclosed in heavy brackets appears in the original patent but forms no part of this reissue specification; matter printed in italics indicates the additions made by reissue.

ABSTRACT OF THE DISCLOSURE This application is a continuation of SN. 719,794 filed Apr. 2, 1968 which is an application for the reissue of US. Patent No. 3,276,857 granted on Oct. 4, 1966 which issued on an application Ser. No. 401,672, filed Oct. 5, J 964 which is a continuation-in-part of Ser. No. 192,084, filed May 3, 1962, now abandoned.

The present invention relates to fertilizer compositions and to an improved method for their manufacture. It pertains more particularly to making a pelleted or compressed fertilizer composition, including a minor proportion of a binder to reduce the rate of aqueous solution and leaching of the more soluble ingredients of the fertilizer. In addition, the compositions of this invention include supplementary barrier materials in addition to the binder, such as waterproofing or water-insoluble ingredients.

Various attempts have been made in the prior art to reduce the water-solubility and leaching rate of inorganic fertilizer materials without rendering the fertilizers incapable of assimilation in growing plants. It has been suggested that particles may be coated exteriorly or mixed with water repellent compositions, etc. According to the present invention, the fertilizer composition is composited of the usual inorganic fertilizing elements such as watersoluble compounds of potassium, phosphorus and nitrogen,. to which are added water repellent materials and supplementary moisture retardants such as water-insoluble or insoluble-barrier type materials.

The problem facing the prior art has been the production of a fertilizer which would slowly release its ingredients to the soil over a prolonged period of time. Various methods have been attempted in order to attain that end. One method known to the art has been coating prills or granules of fertilizer with a thin coating of a water-insoluble material so that the coated fertilizer might lie upon or in the soil for a prolonged period before the coating would 'be eroded and the fertilizer released to the soil. In another method known to the art, particles of nutrients, such as trace elements, were dis- Reissued Nov. 23, 1971 tributed throughout a major portion of water-insoluble binders such as waxes and resins. The effect of this was to form a composition which resembled peanut brittle wherein the peanuts represented the nutrient particles and the caramel represented the binder. Such a procedure would elfect deposition of only small amounts of nutrient per unit weight of the total composition; and such effect might be appropriate for supplying trace-elements, but not for controlled release of fertilizers as explained more fully below.

The present invention is carried out by combining together or co-mixing the necessary ingredients and forming the mixture into firm and dense, water-resistent pills, pellets or briquettes. The composition is preferably formed by first mixing powder or discrete granules of appropriate size, for example, fine enough to pass No. 6 US. sieve, or such as are found in standard, commercial, mixed inorganic fertilizers, with only sufiicient proportions of water-repellent binder material, so as to coat the individual grains or granules with a thin but essentially complete and uniform coating. Additional materials are then incorporated or co-rnixed with the coated or partially coated granules, after which the total mixture is compressed or otherwise compacted into dense, non porous pellets of larger size than the original granules. Various hydrcarbon binder materials may be used such as heavy petroleum residues, asphalts, waxes, blends of these materials, and/or synthetic thermoplastic materials such as various polymers. These may be heated to liquefy them while mixing is accomplished, or they may be dissolved in low boiling solvents which will evaporate to leave the binder in place on the granules. Prior to compression or compaction, however, additional finely divided material is added to further reduce the rate of leaching when. excess quantities of water come into contact with the fertilizer. Suitable materials for this purpose may be water-insoluble inorganic materials or minerals such as calcium carbonate, chalk, finely divided limestone rock, calcined lime, gypsum, crushed phosphate rock which has not been acid treated to make its phosphorus ingredients available to plants, and the like. In addition to these inorganic materials, supplementary quantities of organic substances such as the heavy hydrocarbons and polymeric materials mentioned above may be blended.

It is an object of this invention to provide fertilizer compositions which contain at least and preferably of fertilizer ingredient.

It is an object of this invention to provide a compressed fertilizer composition wherein the fertilizer particles are uniformly coated with the least possible amount of binder material, such as asphalt and/ or wax.

It is another object of this invention to provide a highly compressed fertilizer composition. v

It is an object of this invention to provide a fertilizer composition containing a plurality of discrete fertilizer particles bound together so that no fertilizer particle shall be exposed uncoated at the surface of the composition.

It is an object of the presentinvention to provide a fertilizer composition which is not tacky and resists caking when stored.

It is known in the prior art to prevent ca'king of prills or granules of deliquesoent fertilizers, e.g., ammonium nitrate, by dusting them with powdered kieselguhr. In contradistinction, it is an object of the present invention to form into hard little cakes or pellets a water-soluble fertilizer, e.g., urea which in particulate form has been coated with a hydrocarbon binder and mixed with a powdered mineral, e.g. chalk, by subjecting the mixture to compression at a. moderate temperature. When the shaped compositions of the present invention are heated as high is 250 F. during their manufacture, the resistance of :he fertilizer composition to leaching by water is impaired significantly.

The present invention is directed to a process of prolucing a compressed pellet (pill or briquette) of a slow 'elease fertilizer which has improved storage properties. [he process of this invention in a preferred embodiment :omprises the steps of: (1) intimately mixing 85-95 wt. aercent of finely divided fertilizer with respectively from [-5 wt. percent of a molten binder so as to coat the tertilizer to get a granular, moderately sticky mixture, he mixing of the binder and fertilizer being conducted tt a temperature of between 155-230 F.; (2) intimately nixing 98-90 wt. percent of the fertilizer/binder mixure with respectively 2-10 wt. percent of a powdered, vater-insoluble, inorganic material to get a friable and 'elatively dry mixture; (3) compressing the relatively lry mixture of step 2 at a temperature between 155-212 F. into dense dry pills, briquettes or pellets; and (4) ntimately mixing 95-98 wt. percent of the pellets of itep 3 with respectively 5-2 wt. percent of a molten nicrocrystalline wax of melting point between 130 and !30 F., so as to coat each pellet as completely and lniformly as possible and allowing the coated pellets to :ool. Microcrystalline wax, or microwax is a well-known trticle of commerce that is manufactured in petroleum 'efineries by processes for extracting wax from high- :oiling distillate or residual lubricating oil fractions of rude oil.

The coating of the pellets with the wax is referred o as Wax encapsulation. The encapsulation is dependent m the temperature of the wax and the immersion time )f the pellet in the wax. The time of encapsulation must to of sufficient duration to achieve a coating of substanially complete integrity. Depending upon the efiiciency if contact between the pellets and the wax, the time may ary from 1 second to 1 hour. With intimate contact by tirring, a time of one minute is usually adequate for :ncapsulation. The temperature range for the encapsulaion step is a temperature between 155 F. and 230 F. t is quite obvious that the temperature must not exceed he decomposition temperature of the fertilizer material eing used. Typically, the encapsulation is carried out at 80 F. for a duration of six seconds.

The wax coating on the pellets may be applied either a one step, that is an encapsulation step, or may be ,pplied in a series of steps, that is an encapsulation step ollowed by one or more recapsulation steps. During the ecapsulation steps, less wax is picked up by the pellets han during the encapsulation step. When recapsulating, he pellet, already coated with wax and at about room emperature, is immersed into a bath of molten wax. The ath temperature is maintained above the melting point t the wax, usually at a temperature above about 140 F., .g. about 175 F. This procedure of recapsulation proides for lower net wax pickup.

As is well known to those skilled in the art, plants and to utilize fertilizer such as nitrogen at a relatively teady rate. That is, the nitrogen pickup by the plant from he soil, although it varies over a period of time from lant to plant, does not fluctuate over any great range. herefore it is important to provide the fertilizer to the lant so that the fertilizer is provided at a substantially ontinuous rate. The fertilizer compositions of the instant ivention have been tested and have been shown fertile t substantially the uptake rate of the plant. For instance, orn has a fertilizer uptake of about 3.8, 5.5 and 4.2% er 100 hours and fertilizers encapsulated according to 1e instant invention can provide fertilizer at a rate of bout 6.3, 6.3 and 3.2% per 100 hours, which substanally conforms with the uptake rate of the corn.

A very im ortant difference between coated fertilizer ompositions of the prior art and the compressed fertilzer compositions of the present invention exists in the tanner of release of the water-soluble ingredients; As is well known, the protection of a prior art coated granule or prill of water-soluble fertilizer, even if the coating be complete and very thick, lasts only until the first penetration of the coating or shell permits water to enter, whereupon the whole of the enclosed granule is dissolved. A coating, completely applied, delays the moment of release of fertilizer and release is quick at that moment. The rate of release therefore cannot conform with the rate of uptake by a crop.

By contrast, in the compositions of the present invention, the particles, each encased in a thin shell of coating that may be incomplete, are compressed into a dense pellet, in which they are tightly packed one against others. The particles of fertilizer at the surface of the pellet may be exposed to rapid solution, which can be delayed by encapsulation with wax; but the particles in the next inner layer below the surface do not begin to dissolve until the water to which the pellet is exposed has nearly completely dissolved the particles of the outer layer. In this way alone therate of release of the fertilizer is slower than with a coated fertilizer of the prior art. But there is another phenomenon which slows down the rate of release even further. This is the manner of transferof the interior fertilizer to the surface of the pellet.

The solublefertilizer from the inner particles must be transferred to the surface of the pellet in order to become available in the soil to the plants or crop. As each chamber or empty shell, from which a particle has been dissolved, is connected to an adjoining chamber by only a very small opening, a chain of chambers or a channel is formed. Since the source of water for solution and the position for transfer of soluble fertilizer to the soil are both located at the surface end of this channel, while the particle to be dissolved is located at the interior end of the channel, this particle when dissolved moves as a solution by diffusion through the channel. This is a much slower process than the mass flow of solution from the perforation of the coating of a coated prill.

The materials may be formed into pellets by extruding the mixture while tacky or plastic through suitable dies, or by rolling it into small balls or pellets of appropriate size. However, in order to attain dense ocmpositions, with little or no porosity, it is important to apply pressure. Portions from 2 to 25% by weight of binder may be used, preferably 5 to 15% by weight of powdered or granular inorganic fertilizer. To the pellets may be added from 1 to 20% of the inert or filler materials mentioned above, after which the small pellets may be repelleted into larger pellets. The invention will be further illustrated by the examples which follow.

Example 1 Control over water leaching may be obtained by adding a water-soluble powdered, inorganic solid after mixing an inorganic, Water-soluble fertilizer with a binder to form a coating, prior to compaction. Various inorganic filler materials may be used, as long as they are not reactive with or otherwise detrimental to the fertilizer. However, since most soils where fertilizer is employed also need lime, it is preferred to use gypsum or more preferably calcium carbonate. The latter may be added in the form of powdered chalk or as crushed limestone, for instance. This type of dry filler further slows down the rate of diffusion of water into, and of fertilizer solution out of, the composition of the present invention.

Pellets of small -inch) diameter size and of larger (%-inch) diameter were formed from grains of ammonium sulfate coated with an asphalt-wax binder composition of suitable hardness. An excellent material for this purpose comprises an asphalt of over F., preferably F., softening point combined with a wax, preferably a microcrystalline hydrocarbon wax of fairly high melting point. A 90% asphalt and 10% wax mixture, by

weight, is very satisfactory. With the same binder, a mixture of coated ammonium sulfate and 10% by weight of chalk, essentially CaCO was similarly pelletized. In the first case, the standard fertilizer granules were mixed with the asphalt-wax blend (9 to 1 weight ratio of asphalt to wax) and in the second, the fertilizer granules were mixed with 10% of the binder plus 10% by weight of powdered chalk. Thereafter they were pelletized by extrusion through suitable dies to form short rod-like particles of smooth surface with a fairly hard and firm structure.

The various pellets, coated and pelletized as just described, were subjected to a severe leaching test by placing 5 grams of fertilizer, as computed on total .pellet weight, in a basket and rotating the basket in water until the fertilizer was leached out to the extent of 50% for the larger pellets and of 90% for the small pellets. The degree of leaching was measured by testing the electrical conductivity of the water. The results are tabulated below:

The data show that leaching time was considerably extended, more than double, on the larger pellets and even on the smaller pellets, there was appreciable extension of 33.3% of the time to reach the 90% leaching point.

Obviously, various modifications may be made in the manner of applying the protective binder material and in its composition. Also, the binder may in at least some instances be mixed with the fertilizer by using other expedients than heating, for example, as mentioned above, an evaporable solvent may be used when the binder cannot suitably be melted and applied in liquid form.

It will be understood that several variations in composition may be made, depending on the particular use required and the materials available. In broad terms the composition will normally comprise a substantial proportion of the granular type of fertilizer commonly sold in the agricultural fertilizer industry. As herein used, the term fertilizer is intended to cover the mixed fertilizer composition including compounds of nitrogen, phospho rus and potassium or single compounds of any of the same three elements. To the fertilizer, there may be added the relatively inert mineral filler material in amounts up to 20% but preferably not more than 15% of the weight of the fertilizer material. The binder is employed to bond the whole into pellets substantially larger than the particles of the original fertilizer and of substantially lower water leaching rate, as compared with either the uncoated granules or the coated but uncompacted granules.

The pellets may be die extruded and cut off or broken off while in semi-plastic condition, or they may be rolled up or snow-balled by causing movement of smaller masses to pick upcoatings of binder, subsequently picking up more granular solids, iinely divided inerts, and/or more binder material until the desired pellet size is achieved; but for proper resistance to leaching, it is important to apply pressure. Suitable apparatus for forming either type of pellet, and in fact for forming various shapes, are known and form no part of the present invention. The article of manufacture according to the present invention is formed under a pressure between and 450 pounds per square inch and. is at least inch in its smallest dimension and at most 2 inches in its greatest dimension.

Instead of adding inert inorganic material, the same general objective can be attained in some degree by encapsulating the pellets of fertilizer and binder alone with supplementary binder material. This may be applied as additional coating on the pellet, or it may be mixed with granular material in building up a pellet from smaller pellets. In the latter application, the added bulk material may be granules of fertilizer or may comprise inert material such as the inorganic water insolubles mentioned above.

Example 2 Using two different binders (A) of straight asphalt, penetration grade 30, softening point 130 'F., and (B) a blend of 90% of the same asphalt with 10% of microcrystalline wax of 160 F. melting point, ammonium sulphate was pelletized. In some cases small cylindrical pellets of A -inch diameter were first formed and later made into large cylindrical pellets of %-inch diameter with added binder material and, in some cases, with added fertilizer. These were subjected to accelerated leaching tests in (a) relatively non-porous silt loam soil and (b) relatively porous sandy loam soil with simulated high rainfall conditions, and (c) in a relatively porous soil with very severe, simulated rice culture conditions where water was actuatlly passed through the soil to good subsurface drainage at a rate of one inch of water per day. Under these conditions, the number of 24-hour days required to leach out specified percentages of the fertilizer of the various pellets were determined. The data are tabulated in Table II.

TABLE IL-DAYS REQUIIlt IED TO LEACH AMMONIUM SULPHATE IN VARIOUS LLETED FORMS FROM SOILS Rice culture conditions (1' Water through Diameter Silt loam Sandy loam soil daily) Weight percent of pellet,

Sample number and type binder inch 50% 75% 90% 50% 75% 90% 1 Original V, inch dia. P of 5% B recoated with 3.45% wax.

2 Original Ms inch d a. P of 10% B repelleted into inch dia. with the addition of 4.5% B.

3 Original ,66 inch dia. P of 10% B rcpelletcd into V; inch dia. with the addition of about 50% fresh blend of binder and fertilizer, i.e., to parts of No. 25-1, 45.3 parts of fertilizer and 4.7 parts of binder B.

4 Same as except that an additional 2% wax coat was put on the original A6 inch, 10% B pellets of Sample 5 By extrapolation.

It is clear that the limestone, chalk, or equivalent menioned above can be incorporated in the mixes of the mateials of Table II further to improve the leaching characaristics of the fertilizer as demonstrated in Table I.

Comparison of Sample No. 27-2 with Sample No. 59-3 hows the advantage of encapsulating a pellet of %-inch .iameter with 3.45% wax. Comparison of No. 25-1with Jo. 59-3 shows that by increasing the proportion of bindr from to by weight, the leaching resistance of pellet on -inch diameter is improved to approximate hat of a pellet of %-inch diameter.

Next, comparison of No. 29-2 with No. 25-1 shows hat by repelleting the already improved small pellet of ;inch diameter into a larger pellet of %-inch diameter lith an added small proportion of the same binder, the esistance to leaching is extraordinarily increased manyold. Comparison of No. 29-3 with No. 29-2 shows a imilar but not as great improvement when the repelleting done with an added minor proportion of mixture of ertilizer and binder. Superior results are obtained when he repelleting is done with an added minor proportion of mixture of filler and binder.

Finally, comparison of No. 29-4 with No. 29-3 shows be best results were obtained by encapsulating the repeleted pellets with as little as 2% wax.

The foregoing examples for purposes of illustration were arried out with particles of ammonium sulfate of approx- ."nately mesh, as the fertilizer for the composition of be present invention. Particles of fertilizer may range mm 10 mesh to 200 mesh for the purpose of this invenion. Any normally solid inorganic fertilizer except amuoniurn nitrate can be used in accordance with the present uvention. Ammonium nitrate is objectionable because it orms a sensitive explosive when mixed with hydrocar- -ons. Suitable fertilizers used alone or in mixtures include vmmonium sulfate, ammonium carbonate, monoammoium phosphate, diammonium phosphate, guanidine, cal ium cyanamid, urea, potash, muriate of potash, calcium .itrate, sulfate of potash, potassium phosphate, potassium netaphosphate, ammoniated superphosphate and triple uperphosphate. Urea is a particularly suitable fertilizer If the water-soluble nitrogenous type for use in accordance vith the present invention.

As mentioned above, the compositions of the present uvention are not heated above 230 F. at any time. When ormed by extrusion through dies into cylindrical rods at :ast and at most /2 inch in diameter, the rods are ut into lengths between inch for the smallest diameter vnd 2 inches for the largest diameter; but the compositions re not otherwise broken up or comminuted in any way. he compositions prior to being coated or encapsulated omprise at least 76% by weight and preferably at least 0% by weight of active, water-soluble, inorganic fertilizer articles no larger than 10 mesh. After encapsulation, the roportion of said fertilizer particles is at least 75% and referably at least 85% by weight of the finished article If manufacture.

The binder and the encapsulating material may both e wax with melting point between 130 F. and 175 F. The binder is a thermoplastic hydrocarbon composition zith melting point between 120 F. and 220 F., for exmple, asphalt, or mixtures of asphalt and wax, alone or nodified for plasticity or for melting point with hydroarbon polymers or petroleum resins. The preferred bindr is a blend of asphalt and wax, preferably in proportions etween 10/1 and 4/1. In preparing the composition of be present invention, preferably 85 to 95 parts of the inrganic fertilizer are coated by being contacted intimately iith respectively 15 to 5 parts of molten binder at a temerature not exceeding 230 F. This mixture may be comressed into pellets by extrusion or stamping under presure and the pellets then encapsulated by being sprayed or therwise mixed intimately with 2 to 5% of wax with melting point above 130 F. and molten at a temperaturevbout 180 F.

In a preferred embodiment of the present invention, the mixture of coated fertilizer particles, prior to compaction is intimately mixed with from 10% to 2% of relatively inert filler, preferably powdered calcium carbonate in the form of chalk, limestone, dolomite or calcined lime. In this embodiment, the compositions of the present invention range in proportions of fertilizer/binder/filler, from 76.5/13.5/10 to 93.1/4.9/2. These compositions in compressed shapes, when encapsulated with from 2 to 5% wax, then comprise the same three ingredients in proportions between 75/l3.2/9.8 and 88.4/4.7/l.9. In round numbers, the range of proportions may be stated as 75/13/10/2 to 88/5/2/5 of fertilizer/binder/filler/wax.

What is claimed is:

[1. A dense, slow release pelletized fertilizer composition which is substantially non-tacky and cake-resistant when stored, said composition comprising a major proportion of water-soluble inorganic fertilizer particles and minor proportions of (a) a substantially inert filler and (b) a Water repellent hydrocarbon binder; said particles being at least partially coated with a thin film of said binder; and said filler being distributed throughout said composition so as to improve the leaching resistance of said composition] [2. A composition as defined by claim 1 wherein the fertilizer is ammonium sulfate] [3. A composition as defined by claim 1 wherein said filler is calcium carbonate] [4. A composition as defined by claim 1 wherein said binder is a mixture of wax and asphalt] [5. The composition defined by claim 1 encapsulated with microcrystalline wax] [6. A dense, slow release pelletized fertilizer composition consisting essentially Of (a) at least 75 weight percent of water-soluble inorganic fertilizer particles, (b) about 1 to 20 weight percent of a substantially inert filler and (c) about 1 to 25 weight percent of a water repellent hydrocarbon binder; said particles being a-t least partially coated with a thin film of said binder; and said filler being distributed throughout said composition so as to improve the leaching resistance of said composition] 7. A process for making a dense slow release pelletized fertilizer composition comprising the steps of (a) intimately mixing [at a temperature of about to 230 F.,] suitable water-soluble [inorganic] fertilizer particles and a water repellent hydrocarbon binder so as to substantially coat said particles with said binder, at a temperature above the softening point of the said binder, thereby producing a friable, granular and moderately sticky mixture; (b) intimately mixing the mixture of step (a) with sufiicient substantially inert filler to produce a friable, granular and relatively dry mixture; (c) compressing said dry mixture of step (b), under [relatively high] pressure and at a temperature of about 155 to 212 F., into hard, dense pellets, thereby producing a non-sticky, non-tacky, fertilizer compostion. I

8. A process as defined by claim 7 wherein said hard, dense pellets produced at step (c) are encapsulated in step (d) by substantially coating said pellets with microcrystalline wax.

9. A process for making a dense slow-release pelletized fertilizer composition, comprising the steps of (a) intimately mixing at least 75% by weight of suitable watersoluble fertilizer particles with the balance being a waterrepellent hydrocarbon binder at a temperature above the softening point of said binder so as to substantially coat said particles with said binder, thereby producing a soluble granular moderately sticky mixture; (b) compressing said mixture of step (a) into hard dense pellets and (c) encapsulating the pellets of step (b) by substantially coaring said pellets with wax.

10. A process according to claim 9 further including 1 0 the step of recapsalating the encapsulated particles of step 2,714,062 7/ 1955 Lockrey et a1. 7130 (c) with additional coatings of wax. 2,792,295 5/ 1957 Wright 7164 2,912,317 11/1959 Gloss 71--64 Reference? cted 3,014,783 12/1961 Young 71--64 The followlng references, clted by the Exammer, are 3,085,370 1 5 Br df d et a1 71 2g of record in the patented file of this patent or the original 6 patent.

UNITED REUBEN FRIEDMAN, Primary Examiner STATES PATENTS c. N. HART, Assistant Examiner Kaufman 711 Austin et a1 7164 10 US. Cl. X.-R. Woerther 7164 3 Prince 7164 DA, 64 F 

